1. Field of the Invention
The present invention relates generally to the precision measurement of distance, and more particularly to a long range laser range sensor using a single, frequency-stable source.
2. Description of Related Art
The precision of formation flying control is dependent upon the accuracy of the sensors measuring the relative distances between two bodies. The necessary position knowledge accuracy for future distributed spacecraft missions will be at the micron level and possibly even the nanometer level over inter-spacecraft distances of one to one hundred kilometers, posing unprecedented challenges for ranging precision and dynamic range.
Pulse-based time-of-flight sensors provide centimeter level accuracy, while optical interferometric systems provide nanometer level precision, but with an ambiguity range of approximately one micron. That is, a sensor with an ambiguity range of one micron cannot distinguish the difference between a distance separated by multiples of one micronxe2x80x94a distance of 1.123 microns will be indistinguishable from a distance of 5.123 microns. Thus, there is a gap in the current technology wherein no state of the art technology provides nanometer level accuracy in an ultra-high dynamic range sensor.
FIG. 1 illustrates a heterodyne interferometer. Interferometry is a displacement measurement method in which a path length change between two fiducial points manifests itself as a phase change of the carrier wave. The heterodyne interferometer is a device that uses a low frequency electronic signal having a phase that carries the optical carrier phase. Because the phase of the low frequency heterodyne signal can be measured very precisely, heterodyne interferometers are used for high-precision applications.
A laser 100 that is selected to have a stable and predictable optical signal over time provides a fundamental optical signal v that is provided to a frequency shifting unit 120. The fundamental optical signal is converted into two distinct optical signals with closely separated frequencies, the target frequency (v+fT) and the local frequency (v+fL), by the frequency shifting unit 120. The two optical signals form what is known as a xe2x80x98heterodyne pair.xe2x80x99 Although shown in FIG. 1 as a separated pair of signals for clarity, the two signals can actually be incident upon one another with orthogonal linear polarizations. The two optical signals, target and local, are introduced to a non-polarizing beam splitter 140 that transmits part of the signals and deflects the remaining portion of the signals to a reference photodetector 150. The two deflected signals travel the exact same distance and when combined provide a reference condition to measure the displaced signal. The transmitted signals are directed to a polarizing beam splitter (PBS) 160 that distinguishes between the target and local frequency signals. The local frequency signal is transmitted through the PBS to a signal photodetector 190 while the target frequency signal is directed to a first retro-reflector 170 corresponding to a fixed position for measurement. The target frequency signal is then reflected through the PBS 160 to a second retro-reflector 180 on the displaced vehicle (not shown), and then reflected to the signal photodetector 190. The target frequency signal and the local frequency signal received by the signal photodetector 190 differ in the distance traveled by 2L, where L is the distance between the two retro-reflectors 170,180.
Small changes in L cause a measurable change in the phase of the target frequency signal. When the target frequency signal and the local frequency signal are mixed at the photodetectors 150,190, the phase of the output signal at frequency (fTxe2x88x92fL) is indicative of the relative phase of the two input signals. The phase meter 195 compares the phase of the heterodyne signal from the signal photodetector 190 with that generated by the reference photodetector 150. The measured phase xcfx86 is equal to the optical phase delay between the target signal and the local signal and contains the desired distance information
xcfx86≈4xcfx80vL/c
This equation has solutions for L that repeat at intervals of the ambiguity range, xcex94L≈c/2v, or 0.65 xcexcm for a 1.3 xcexcm laser. Standard heterodyne metrology does not distinguish between the many possible solutions, and is therefore only useful as a differential metrology system that measures changes in distance.
This limitation can be overcome with a two-source interferometer, where standard heterodyne phase measurements are made using two different optical frequencies, v1 and v2:
xcfx861=4xcfx80v1L/c
xcfx862=4xcfx80v2L/c
The difference in phases is given by the expression
xcfx861xe2x88x92xcfx862=4xcfx80(v1xe2x88x92v2)L/c, or
xcfx861xe2x88x92xcfx862=4xcfx80FSL/c,
where the xe2x80x9csyntheticxe2x80x9d frequency FS replaces the difference in the actual frequencies. The ambiguity range for this difference measurement is increased to xcex94L≈c/2vFS, and the range resolution is "sgr"L=xcfx81c/2FS, where xcfx81 is the phase resolution in fractions of a cycle. Two source or two xe2x80x9ccolorxe2x80x9d interferometers use two separate lasers to generate v1 and v2. However, there are major drawbacks to this approach.
Traditionally, two color absolute interferometers are implemented with two lasers detuned from each other by the required frequency. The required frequency detuning can be achieved with semiconductor lasers, but the semiconductor lasers do not have the narrow linewidth and high frequency stability required for range measurements over long target distances. A metrology scheme using direct intensity modulation of the optical carrier signal would require detection and processing of high frequency signals (approximately 120 GHz). However, efficient photodetectors that can operate at these frequencies do not currently exist and therefore the detection of this modulation, if possible at all, would require high optical power. For many proposed applications of the sensor of the present invention, a constraint that the sensor operate with low received optical power precludes this possibility. Further, methods for dealing with high levels of self-interference have been developed for heterodyne interferometers, but presently there are no comparable methods that exist for direct modulation systems.
The present invention is an improved distance measuring interferometer that includes high speed phase modulators and additional phase meters to generate and analyze multiple heterodyne signal pairs with distinct frequencies. Modulation sidebands with large frequency separation are generated by the high speed electro-optic phase modulators requiring only a single frequency stable laser source and eliminating the need for a first laser to be tuned or stabilized relative to a second laser. The combination of signals produced by the modulated sidebands is separated and processed to give the target distance. The resulting metrology apparatus enables a sensor with submicron accuracy or better over a multi-kilometer ambiguity range.